Perovskite photovoltaic device

ABSTRACT

A photovoltaic device, comprises (1) a first conductive layer, (2) an optional blocking layer, on the first conductive layer, (3) a semiconductor layer, on the first conductive layer, (4) n light-harvesting material, on the semiconductor layer, (5) a hole transport material, on the light-harvesting material, and (6) a second conductive layer, on the hole transport material. The light harvesting material comprises, a pervoskite absorber, and the second conductive layer comprises nickel. The semiconductor layer tray comprise TiO 2  nanowires. The light-harvesting material may comprise a pervoskite absorber containing a psuedohalogen.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under CBET-1150617awarded by the National Science Foundation. The government has certainrights the invention.

BACKGROUND

Photovoltaic (PV) systems are systems that convert light intoelectricity. All photovoltaic systems share a few common parts. Allphotovoltaic systems include a light-harvesting element, acharge-separating element, a charge-transporting element(s), and acharge collecting element(s).

A perovskite solar cell is a type of solar cell which includes aperovskite absorber (that is, light-harvesting element). A perovskiteabsorber has an ABX₃ formula, where A is a metal atom such as lead ortin, B is a counter ion (typically an alkyl ammonium compound), and X isa halide (F, Cl, Br, or I). The name “perovskite” refers to the crystalstructure of the absorber materials, which has a perovskite structure.The most commonly studied perovskite absorber is methylammonium leadtrihalide (CH₃NH₃PbX₃, where X is a halogen ion such as I—, Br—, Cl—),with a bandgap between 2.3 eV and 1.57 eV depending on halide content.Formamidinium lead trihalide (H₂NCH₃NH₃PbX₃) is a recently studied newermaterial which shows promise, with a bandgap between 2.23 eV and 1.48 V.

These hybrid organic-inorganic solid state solar cells with perovskitestructured CH₃NH₃PbI₃ as active layer have recently been reported withover 15% efficiency,^(1,2) which emerges as the most promising candidatefor the next-generation solar cells. As a game changer in photovoltaics,perovskite-type material exhibits striking excellence in both lightabsorption³ and charge transport (1069 nm electron diffusion length and1213 nm holes diffusion length).⁴ Despite its aura, the dark side ofthis rising star should not be ignored such as the stability issue ofCH₃NH₃PbI₃,⁵ the use of environment-hazardous lead: the cost of complexorganics as hole-blocking layer,^(7,8) and the use of expensive noblemetals as back cathode. In order to maximize the attainable open circuitvoltage (V_(oc)), low chemical potential, namely, high work functionnoble metals, such as gold^(2,8,10) and silver,¹ are generally used asback cathode. Thermal evaporation of gold is a very costly and wastefulprocess because only a tiny portion of the gold is eventually depositedonto the devices. Therefore, replacing gold with earth-abundant elementsas the cathode in perovskite solar cells while still retaining theirhigh V_(oc) and energy conversion efficiency is a pivotally criticalstep toward the cost-effective production of perovskite solar cells.

SUMMARY

In a first aspect, the present invention is a photovoltaic device,comprising (1) a first conductive layer, (2) an electron blocking layer,on the first conductive layer, (3) a semiconductor, layer, on theelectron blocking layer, (4) a light-harvesting material on thesemiconductor layer, (5) a hole transport material, on thelight-harvesting material, and (6) a second conductive layer, on thehole transport material. The light-harvesting material comprises aperovskite absorber, and the second conductive layer comprises nickel orcobalt.

In a second aspect, the present invention is a photovoltaic device,comprising (1) a first conductive layer, (2) an electron blocking layer,on the first conductive layer, (3) a semiconductor layer, on theelectron blocking layer, (4) a light-harvesting material, on thesemiconductor layer, (5) a hole transport material, on thelight-harvesting material, and (6) a second conductive layer, on thehole transport material. The light-harvesting material comprises aperovskite absorber, and the semiconductor layer comprises TiO₂nanowires.

In a third aspect, the present invention is a method of forming thephotovoltaic device of any one of the preceding claims, comprisingforming the first conductive layer, and the electron blocking layer onthe first conductive layer; forming the semiconductor layer, on thefirst conductive layer: applying the light-harvesting material, onto thesemiconductor layer, forming the hole transport layer, on thesemiconductor layer; and forming the second conductive layer, on thehole transport layer.

In a fourth aspect, the invention is a photovoltaic device, comprising(1) a first conducive layer, (2) an electron blocking layer, on thefirst conductive layer (3) a semiconductor layer, on the electronblocking layer, (4) a light-harvesting material, on the semiconductorlayer, (5) a hole transport material, on the light-harvesting material,and (6) a second conductive layer, on the hole transport material. Thelight-harvesting material comprises a perovskite absorber containing apseudohalogen.

Definitions

A “perovskite solar cell” or “perovskite-type solar cell” is a solarcell which includes a perovskite absorber as the light-harvestingelement.

A “perovskite absorber” is a compound of formula ABX₃, where A is ametal atom such as lead or tin, B is a counter ion (typically an alkylammonium compound), and X is a halide (F, Cl, Br, or I) or pseudohalide(such as SCN), which forms crystals of the perovskite structure.Examples include CH₃NH₃PbX₃ and H₂NCH₃NH₃PbX₃ and CH₃NH₃Pb(SCN)₂I

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a schematic diagram of a perovskite solar cell with listedcomponents being preferred: a fluorinated tin oxide (FTO) anode coveredwith a electron blocking TiO₂ thin film, ruble TiO₂ nanowires,spin-coated CH₃NH₃PbI₃ layer, or spin-coated CH₂NH₃Pb(SCN)₂I layer andsprit-MeTAD layer, followed by a sputtered nickel cathode.

FIG. 1B shows the energy diagram, of the device of FIG. 1A.

FIG. 2a is a SEM image of TiO₂ nanowires on a FTO substrate (insetmagnified SEM image).

FIG. 2b shows HR-TEM images of TiO₂ nanowires.

FIG. 2c is a SEM top-view image of a cell near the edge of a goldcathode.

FIG. 2d is a cross-section SEM image of a cell with gold as cathode(inset: digital photo of an actual device with six cells).

FIG. 2e is a SEM top-view image of a cell near the edge of a nickelcathode.

FIG. 2f is a SEM cross section image of a cell with nickel as cathode(inset; digital photo of an actual device with six cells).

FIG. 3 is a graph of J-V characteristic under illumination (1 sun, 1.5global air mass) and in dark for gold-backed and nickel-back cells. Bothcells use 900 nm TiO₂ nanowires on FTO as anode for a fair comparison.

FIG. 4 is a graph of J-V curves of cells using 900, 300, and 1200 nmTiO₂ nanowires as anode and 80 nm thick gold as back electrode.

FIG. 5 is a graph of J-V curves of a cell using 1200 nm TiO₂ nanowiresas anode and 150 nm thick nickel as back cathode.

FIG. 6A is a graph of J-V curves for nickel-cathoded cells having 150 nmthick Ni measured on day 1, day 2, and day 5 Between each measurement,the device is stored in a desiccator.

FIG. 6B is a graph of J-V curves for nickel-cathoded cells having 300 nmthick Ni measured on day 1, day 2, and day 5. Between each measurement,the device is stored in a desiccator.

FIG. 7 is a graph of photovoltage rising transients of a 900 nm richnanowire (NWs) photoanodes devices with 80 nm Au as cathode and a 900 nmTiO₂ NWs photoanodes device with 300 nm Ni as cathode.

FIG. 8a is a graph of the moisture-tolerance test of conventionalCH₃NH₃PbI₃.

FIG. 8b is a graph of the moisture-tolerance test of CH₃NH₂Pb(SCN)₂I.

FIG. 9 is an X-ray diffraction (XRD) patterns of CH₃NH₃Pb(SCN)₂Icomparison to the conventional CH₃NH₃PbI₃, along with Pb(SCN)₂ andCH₃NH₃I (MAO.

DETAILED DESCRIPTION

The present invention makes use of the discovery that noble metals maybe replaced with nickel, as the back cathode. In the guest for high workfunction metals, noble metals such as gold, platinum, or silver oftencome up as the routine choice. The present invention break down thisstereotype selection rule for perovskite-type solar cells using nickelas the back cathode. Nickel has a unit price less than 0.03% e that ofgold. The present invention also makes use of the discovery that TiO₂nanowires may be used to efficiently collect electrons generated fromthe perovskite light harvesting elements in perovskite solar cells. Thepresent invention also makes use of the discovery that by replacing twoiodide with two pseudohalogens, that is, thiocynide ions, (—SON), a newperovskite material CH₃NH₃Pb(SON)₂I has been developed that exhibitsmuch better moisture-tolerance than the conventional CH₃NH₃PbI₃. Thethree discoveries may be used independent of the other, or preferablyused together.

FIG. 1A illustrates a solar cell, 7, with preferred materials listed;other materials for each of the different layers may be used instead ofthose listed. The solar cell includes a first conductive layer, 6; anelectron blocking layer, 5, on the electron blocking layer, asemiconductor layer, 4, on the first conductive layer a light-harvestingmaterial, 3, on the semiconductor layer a hole-transporting material, 2,on the semiconductor layer; and a second conductive layer, 1, on thehole-transporting material.

Preferably, the first conductive layer is transparent, so that light maypenetrate one side of the device and reach the light-harvesting materialOptionally, the first conductive layer may be on a substrate. Examplesof substrates include glass, quartz and transparent polymeric materials,such as polycarbonate. Examples of transparent conductive layers includeindium-tin oxide, fluorinated tin oxide, and aluminum-zinc oxide.Graphene may also be used as the first conductive layer. The firstconductive layer may also be formed as a composite material and/orformed as multiple layers. For example, a planar substrate of glass maybe coated with a layer of fluorinated tin oxide, and fine particles offluorinated tin oxide applied to die surface and sintered together toprovide the substrate and first conductive layer.

In an alternative configuration, such as that described in PatentApplication Publication, Pub. No. US 2011/0220192, the first conductivelayer, with the semiconductor layer and light harvesting material, areon the support, but spaced away from the electrode and second conductinglayer, and not in direct electrical contact therewith. In operation ofthis alternative configuration, light does not need to travel throughthe first conductive layer, so a non-transparent conductive Layer may beused, for example a metal such as nickel, gold, silver or platinum, or aconductive oxide, such as electrically conductive titanium suboxides.

The optional blocking layer, which serves to bind defective sites andsuppress back electron transfer, and may have a different compositionthan the semiconductor layer, and is preferably a transparent insulatingmaterial, for example titanium dioxide (TiO₂), magnesium oxide (MgO),aluminum oxide (Al₂O₃), zirconium oxide (ZrO₂), boron nitride (BN),silicon oxide (SiO₂), diamond (C), barium titanate (BaTiO₃), andmixtures thereof. The blocking layer may also be formed of a transparentsemiconductor material and preferably is an n-type semiconductor, forexample titanium dioxide (TiO₂), zinc oxide (ZnO), zirconium, oxide(ZrO₂), tungsten oxide (WO₃), molybdenum oxide (MoO₃), lead oxide (PbO),and mixtures thereof, or mixtures thereof with a transparent insulatingmaterial. It is important that the blocking layer be both conformal andcompact.

The optional blocking layer preferably has a thickness of at most 2 nm,or may be present in an amount of at most 10 atomic layers. It may alsobe present as islands on the surface of the semiconductor layer, inwhich case the thickness may be expressed as an average thickness acrossthe semiconductor layer, for example as less than one atomic layer.

The semiconductor layer, which is n-doped or n-type, may be atransparent semiconductor, such as titanium dioxide (TiO₂), zinc oxide(ZnO), zirconium oxide (ZrO₂), tungsten oxide (WO₃), molybdenum oxide(MoO₃), lead oxide (PbO) or mixtures thereof. Preferably, thesemiconductor layer has a thickness of at most 100 nm, for example 1 to100 nm, including 5, 10, 15, 20, 25, 30, 35, 40, 46, 50, 55, 60, 66, 70,75, 80, 85, 90 and 95 nm. If the semiconductor layer is notintrinsically formed as an n-type semiconductor, such as is the casewith TiO₂, is may be chemically n-doped.

The semiconductor layer may be formed by physical vapor deposition suchas evaporation or sputtering, or by chemical deposition, such as atomiclayer deposition, or by forming a thin layer of a precursor which isthen decomposed to form the semiconductor layer. Electrochemicaldeposition or deposition from solution, may also be used in the case ofconductive polymers. The thickness may be controlled by the amount ofsemiconductor initially deposited, or by removing depositedsemiconductor by etching, such as chemical etching. The semiconductorlayer may also be formed by applying a dispersion of fine particles ofthe semiconductor dispersed into a fluid, for example particles have anaverage diameter of 5 to 100 nm, including 10, 20, 30, 40, 50, 60, 70,80 or 90 nm, dispersed in water, or an organic solvent for examplealcohols such as methanol or ethanol, or mixtures thereof. Sintering maybe desirable to remove the solvent and/or improve the contact betweenthe semiconductor layer and the first conductive layer, or to improvethe crystallinity of the semiconductor layer. It is important that thesemiconductor layer both conformal and compact. Ideally, the contactbetween the first conductive layer and the semiconductor layer should bean ohmic contact.

Atomic layer deposition may be carried out by chemical reaction of twocompounds which react to form the semiconductor layer. The structureonto which the semiconductor layer is to be deposited is exposed tovapors of the first of the two chemicals, and then exposed to the vaporsor gasses of the second of the two chemicals. If necessary, the exposureand/or reaction may be carried out at elevated temperatures. In someinstances, byproducts of the reaction may need to be removed beforerepeating the process by washing, evacuation, or by the passage of aninert gas over the structure. The process may be repeated until thedesired thickness of the semiconductor layer is formed. For example, inthe case of the transparent oxide semiconductors, which are typicallycompounds of a metal and oxygen, the first chemical may be a halide,such as a chloride, bromide or iodide, an oxychloride, oxybromide oroxylodide, organometallic compounds, alkoxides of the metal and otherceramic precursor compounds (such as titanium isopropoxide), as well asmixtures thereof. The second chemical may be water (H₂O), oxygen (O₂and/or O₃) or a gaseous oxidizing agent, for example N₂O, as well asmixtures thereof. Inert gasses, such as helium, argon or nitrogen may beused to dilute the gasses during, the process.

In a preferred alternative embodiment, the semiconductor layer iscomposed of TiO₂ nanowires. The nanowires may be prepared bysolvothermal method with controllable length-to-diameter ratio and arewell separated.¹³ Preferably, the length of the TiO₂ nanowires is400-1100 nm, more preferably 600-1000 nm, including 700, 800 and 900 nm.

The light-harvesting material is a perovskite absorber, a compound offormula ABX₃, where A is a metal atom such as lead or tin, B is acounter ion (typically an alkyl ammonium compound), and X is a halide(F, Cl, Br, or I) or pseudohalide such as SCN (thiocynide ion), whichforms crystals of the perovskite structure. Examples include CH₃NH₃PbX₃and H₂NCH₃NH₃PbX₃. Preferably, the light-harvesting material is appliedby spin-coating so that it fills spaces on and in the semiconductorlayer,

The hole-transporting material may be a solid p-type semiconductor, forexample CuI, CuSCN CuAlO₂, NiO, and mixtures thereof, as well as p-dopedconductive polymers. Conductive polymers include poly(acetylene)s,poly(pyrrole)s, poly(thiophene)s, polyanilines, polythiophenes,polyp-phenylene sulfide), poly(para-phenylene vinylene)s (PPV) and PPVderivatives, poly(3-alkylthiophenes), polyindole, polypyrene,polycarbazole, polyazulene, polyazepine, poly(fluorene)s, andpolynaphthalene. Other examples include polyaniline, polyanilinederivatives, polythiophene; polythiophene derivatives, polypyrrole,polypyrrole derivatives, polythianaphthene, polythianaphthanederivatives, polyparaphenylene, polyparaphenylene derivatives,polyacetylene, polyacetylene derivatives, polydiacethylene,polydiacetylene derivatives, polyparaphenylenevinylene,polyparaphenylenevinylene derivatives, polynaphthalene, andpolynaphthalene derivatives, polyisothianaphthene (PITN),polyheteroarylenvinylene (ParV), in which the heteroarylene group can befor example thiophene, furan or pyrrol, polyphenylene-sulphide (PPS),polyperinaphthalene (PPN), and polyphthalocyanine (PPhc), and theirderivatives, copolymers thereof and mixtures thereof. As used herein,the term derivatives means the polymer is made from monomers substitutedwith side chains or groups. P-doping of the solid semiconductor and theconductive polymers may be carried out chemically, if necessary, forexample by treatment with an oxidizing agent, such as oxygen, fluorineor iodine, or by electrochemical oxidation. A preferredhole-transporting material is spiro-MeOTAD(2,2′7,7′-tetrakis(N,N-di-p-methoxyphenyl amine)-9,9′-spirobifluorene).

A second conductive layer is in contact with the hole-transportingmaterial, and is preferably formed of a highly conductive and chemicallyunreactive material, for example gold, platinum, or metallic alloys.Preferably, the second conductive layer is nickel or a nickel alloy. Thesecond conductive layer may be present on a third conductive layer,which may be formed of any conductive material. The second conductivelayer serve to transport electrons back to the hole-transportingmaterial, thus completing the electrical circuit. The second conductivelayer is preferably on a support, which may be formed of any solidmaterial, such as plastic, glass or metal. Preferably, the secondconductive layer is formed by evaporation or sputtering.

As shown in FIG. 1A, using the preferred material listed on theillustration, the cell contains about 900 nm high rutile TiO₂ nanowire(NWs) array on FTO (fluorinated tin oxide) glass as the photoanode,¹³which is filled with a layer of spinning-coated CH₃NH₃PbI₃, a 220 nmthick spinning-coated Spiro-MeOTAD(2,2′7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene) ashole transport material (HTM), capped by a sputtering-coated nickel filmas the cathode.

EXAMPLES

Preparation of the Photo odes having Rutile TiO₂ Nanowire Arrays on FTO.

The TiO₂ nanowire arrays are fabricated via a solvothermal methodreported previously. In a typical process, FTC substrates coated with alayer of TiO₂ thin film were loaded into a sealed Teflon-lined stainlesssteel reactor filled with 6 mL of 2-butanone, 6 mL of 37% hydrochloricacid, and 0.5 mL of tetrabutyltitanate, and the mixture was then kept at200° C. for 40 min. The obtained substrates with nanowire arrays werecleaned by dipping in a H₂O₂ (30 wt %)/NH₄OH (25 wt %) (10:1 of volumeratio) solution for 10 min., followed by annealing, at 450° C. for 30min under an O₂ atmosphere. The obtained electrodes were then soaked in40 mM TiCl₄ solution at 65° C. for 1 h, followed by rinsing with DIwater. The TiO₄-treated TiO₂ nanowires were annealed at 500° C. for 30min.

Synthesis of CH₃NH₃I.

19.5 mL of methylamine (40 wt % water solution) was dropwisely added to32.3 mL of hydrolodic acid (57 wt % in water, Aldrich) in an ice bathand under stirring for 2 h. The precipitate was recovered by extractingthe mixture in a rotary evaporator at 50° C. The yellow/brown productwas filtered and washed by diethyl ether to yield white powders, whichwas further purified by recrystallization in ethanol and diethylether.¹⁴ The nuclear magnetic resonance (NMR, Bruker Avance III 500 MHz)spectrum of the product in d6-DMSO can be found in Figure S1 of theSupporting Information²¹.

Preparation of precursory CH₃NH₃PbI₂ Solution.

0.29 g (0.63 mmol) of PbI₂ and 0.1 g (0.63 mmol) of CH₃NH₃I eredissolved in 0.5 mL of γ-butyrolactone solution at 60° C.¹⁵

Preparation of precursory CH₃NH₃Pb(SCN)₂I Solution.

CH₃NH₃Ph(SCN)₂I precursory solution is prepared as following: 0.283 gPb(SCN)₂ and 0.15 g CH₃NH₃i were dissolved in 0.6 ml DMF under stirringat 60° C. for 3 hours.

Preparation of HTM Solution.

92 mg of2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene(spiro-MeOTAD), 7.2 mg of Li bis-trifluoromethane sulfonimide, and 12 mgof 4-tert-butylpyridine were dissolved in 1 mL of chlorobenzene.

Device Fabrication.

The precursory solution of CH₃NH₃PbI₃ or precursory solution ofCH₃NH₃Pb(SCN)₂I was spin coated on the TiO₂ nanowire photoanodes at 2000rpm for 1 min in ambient conditions. The deposited CH₃NH₃PbI₃ orCH₃NH₃Pb(SCN)₂I film was dried in Ar at 105° C. for 10 min, followed byspinning coating HTM at 2500 rpm. Nickel was sputtered on the top of theHTM layer through a homemade aluminum mask to form the nickel pads asback cathodic electrode. Au films were deposited with an Edward 306Athermal evaporator with base pressure of 6×10⁻⁶ Torr. Nickel thin filmswere deposited using the magnetron sputtering technique. The rotarymagnetron sputtering system was customized by Denton Vacuum with DCpower supplies. The system has sputtering targets set vertically. Thebased pressure of the deposition was 1×10⁻⁶ Torr, and the depositionpressure was 3 mTorr of Ar. The distance of substrate-to-target is 7 cm.The Ni target was ignited at the power of 80 W and presputtered for 5min. The samples were then moved to the sputtering zone in a horizontalcycling swiping manner (1 nm/loop, 6 s/loop). The growth rate wascalibrated using SEM cross-section measurements. The temperature on thesubstrate surface is monitored with a set of irreversible temperatureindicators (Cole-Palmer) ranging from 65 to 135° C. After sputteringunder above conditions, none of the temperature labels turn black,suggesting that the temperature on substrate is below 65° C. duringsputtering.

Structure Characterization and Other Measurement

The morphologies and microstructures of the samples werecharacterization using a Hitachi-S4800 FE-SEM or a Tescan SEM (modelVega it SSH). The TEM and HR-TEM images were taken using a Tecnai F20(FEI Hillsboro, Oreg.) microscope at, an accelerating voltage of 200 kV.The sheet resistance measurement is conducted using a Jandel four-probesheet resistance measurement system,

Photovoltaic Performance Measurement.

The current density-voltage (J-V) curves were collected using apotentiostat (Gamry Reference 600) and the devices were illuminated atone sun 1.5 air mass global (AM G) spectrum provided by a solarsimulator (Photo Emission inc., Camarillo, Calif., model SS50B). It isreported that hysteresis appears in J-V measurement at low voltage scanrate (<150 mV/s).¹⁶ This is seen in our initial study, and therefore, tominimize hysteresis, we set the voltage sweeping rate at 200 mV/s with astep of 2 mV, which is a fairly optimized setting.¹⁷ This setting isapplied to all J-V measurements in this work for fair comparisonsbetween different devices. All devices were measured under the samemeasurement conditions as described above on day 1 (the day that thedevice was prepared), day 2, and day 5 to see if samples performanceundergo any significant changes, at least within the first 5 days.Between each measurement the samples are stored in a desiccator. Theopen-circuit photovoltage transient was measured on Gamry Reference 600in the Chronopotentiometry mode. An ultrafast optical shutter is used(UniBlitz model LS6, 700 is from full closure to full open, aperture is6 mm in diameter with a shutter driver model D 122) to control theincident sunlight. A commercial solid-state silicon photodiode (OSRAMOpto Semiconductor, SPX 61, response time 20 ns, spectral range 400-1100nm, and active area 2.65 mm×2.65 mm=7 mm²) is used as a reference toverify the measurement limit of our setup (Figure S2 in SupportingInformation²¹).

Results and Discussion

FIG. 2a is the scanning electron microscope (SEM) image of the ˜900 nmTiO₂ nanowires array grown almost vertically on the FTC substrate. Thediameter of TiO₂ nanowires is about 30-40 nm (see inset of FIG. 2a ).The high-resolution transmission electron microscope (HRTEM) image inFIG. 2b suggests a single crystal feature of the nanowires. The (110)lattice crystal plane with a fringe spacing of 0.325 nm indicates thatthese nanowires are rutile TiO₂. FIG. 2c is the top-view SEM image ofthe cell near the edge (along the yellow dashed line) of a gold cathode.The lower left portion of FIG. 2c is the morphology of TiO₂ nanowirephotoanode after spincoating with CH₃NH₃PbI₃ or CH₃NH₃Pb(SON)₂I andspiro-MeOTAD, while the upper right portion of FIG. 2c is the morphologyafter gold evaporation. It can be clearly seen that thermal evaporationof gold does not change the morphology of the underneath layers. FIG. 2dis the cross-section SEM image of a device with gold as cathode, inwhich each layer including the wrapped TiO₂ nanowires by CH₃NH₃PbI₃, orthe CH₃NH₃Pb(SCN)₂I layer, the spiro-MEOTAD layer, and the gold layer,can be identified. The thickness of Spiro-MeOTAD is about 220 nm, whichis the optimized thickness that yields the best of we can achieve. Theinset of FIG. 2d is the photo of the device with six gold cathodes, FIG.2e is a top view of the cell near the edge (along the dashed line) of anickel cathode. Unlike the shape edge found in the thermally evaporatedgold cathode, the sputter nickel does not give a very sharp edge. Theupper right part of FIG. 2e is covered with nickel, while the lower leftpart is intact. Nonetheless, it can still be seen that the morphology ofthe CH₃NH₃PbI₃ or CH₃NH₃Pb(SCN)₂I layer and spiro-MeOTAD layer remainsthe same after sputtering compared with the parts that are not coatedwith nickel, indicating that the nickel sputtering process does notchange the morphology of the spin-coated CH₃NH₃PbI₃ or theCH₃NH₃Pb(SCN)₂I layer and Spiro-MeOTAD layers under our optimizedoperational parameters. The inset of FIG. 2f shows six nickel cathode.The diameter for all the cathodes is 3 mm.

FIG. 3 shows the photocurrent density versus photovoltage (J-V) curvesof cells using 900 nm TiO₂ nanowires as photoanodes and using $0 nm goldfilm, 150 nm nickel film, and 300 nm nickel film as cathodes,respectively. The photovoltaic performance of the cells are summarizedin Table 1. It can been seen that the cell using 300 nm nickel film ascathode exhibits comparable energy conversion efficiency (η=10.4%) tothat of the cell using 80 nm gold film as cathode (η=11.8%). However,the cells using 150 nm nickel film as the cathodes exhibits a notabledrop in efficiency (η=7.7%).

TABLE 1 Photovoltaic Performance of Solar Cells with Ni and Au as BackCathodes cathodes J_(sc) V_(oc) of cells (mA m⁻²) (mV) FF η (%) R_(sh)(Ω) R_(s) (Ω) Ni 150 nm 21.4 752 0.50 7.7 11794.6 264.4 Ni 300 nm 20.6830 0.61 10.4 13851.4 103.2 Au 80 nm 21.7 792 0.68 11.6 14160.9 81.5

To troubleshoot the source of energy loss in 150 nm nickel cathodedcells, short circuit current density (J_(sc)) for ail cells are firstcompared. However, both cells exhibit almost the same 0.6, about 20mA/cm⁻². Furthermore, V_(oc) of the cell using 150 nm nickel as cathodeis 0.752 V, only slightly lower than that of the cells using 80 nm goldor 300 nm Ni film as cathode. Then, compared to the fill factor (FF) or68% for the cell using gold as cathode, it is the relatively low FF(50%) found in the cell using nickel as cathode that is responsible forits low conversion efficiency. Since there is no significant differencein V_(oc) and J_(sc) in comparison to the other two cells, we attributethe low FF in cell with 150 nm nickel cathode mainly to its high seriesresistance, indeed, shunt resistance (R_(sh)) at J_(sc) for 150 nmnickel-backed cell is 11 794Ω, which is about 80% of that forgold-backed cell, while the series resistance (R_(s)) at V_(oc) for Incnickel-backed cell is 264Ω, over 3 times higher than that of thegold-backed cell. The relations between current, fill factor, and shuntresistance are shown by the following equations¹⁸

$\begin{matrix}{{FF}_{SH} = {{FF}_{0}\left( {1 - \frac{1}{R_{SH}}} \right)}} & (1) \\{I = {I_{L} - {I_{0}{\exp \left\lbrack \frac{qV}{nkT} \right\rbrack}} - \frac{V}{R_{SH}}}} & (2)\end{matrix}$

where I is the cell output current, I_(L) is the light generatedcurrent, V is the voltage across the cell terminals, T is thetemperature, q is the charge of an electron, k is Boltzmann constant, nis the ideality factor, and R_(SH) is the cell shunt resistance. Theidea cell which is not affected by any resistance is denoted by I₀ andFF₀.

The relations between current, fill factor, and series resistance areshown by eqs 3 and 4.¹⁸ R_(s) is the series resistance.

$\begin{matrix}{{FF}_{s} = {{FF}_{0}\left( {1 - R_{s}} \right)}} & (3) \\{I = {I_{L} - {I_{0}{\exp \left\lbrack \frac{q\left( {V + {IR}_{s}} \right)}{nkT} \right\rbrack}}}} & (4)\end{matrix}$

Equations 1-4 can be combined to briefly estimate the respective impactfrom series resistance and shunt resistance on FF in comparison to anideal solar cell. The simulated result suggests that the loss of FF inthe cell with a nickel cathode is mainly caused by the high seriesresistance (Figure S2).²¹ As supporting evidence, four-probe sheetresistance of the 80 nm thick gold cathodes is measured to be 0.2-0.4Ω/square, in contrast to 90-100 Ω/square for the 150 nm thick nickelcathode. The two-probe measurement on the edge-to-edge resistance ofgold pads is typically about 0.5Ω, in contrast to ˜150Ω for nickel pads.Indeed, cells with a 100 nm thick nickel cathode, i.e. even higherseries resistance, yield even lower FF as shown in Figure S3²¹ and TableS1²¹.

As we further increased the thickness of the nickel film to 300 nmthick, the FF of the cell increases to 0.61 and the conversionefficiency reaches 10.4%. The two-probe measurement on the edge-to-edgeresistance of the 300 nm thick nickel pads is measured to be 3Ω, and thefour-probe sheet resistance of the 300 nm thick nickel cathodes ismeasured to be 3-5 Ω/square.

The effect of the length of the TiO₂ nanowires was also studied. The J-Vcurves of cells using 300, 900, and 1200 nm TiO₂ nanowires asphotoanodes and 80 nm thick gold as back electrode cells are shown inFIG. 4 and Table 2. The result suggest that that 900 nm TiO₂, nanowiresgive the best result.¹⁹

TABLE 2 Photovoltaic Performance of Solar Cell with 300, 900, and 1200nm TiO₂ Nanowires as Photoaonde and 80 nm Gold as Back Cathode J_(sc)V_(oc) NWs length for cells (mA m⁻²) (mV) FF η (%) 300 nm 19.3 850 0.6310.4 900 nm 21.7 792 0.68 11.6 1200 nm  9.1 722 0.73 4.7

Further increase of the length of the nanowires deteriorates the cellperformance as shown by the device using 1200 nm TiO₂ nanowires as anodeand 150 nm Ni as cathode, which only reach 4.1% in efficiency (FIG. 5).This trend agrees with the Au-cathoded cells.

The stability of the photovoltaic performance of the cells using nickelcathodes are also studied as shown in FIGS. 6A and 6B and summarized inTable 3. FIGS. 6A and 66 respectively show the J-V curves of the cellsusing 150 nm Ni and 300 nm Ni film as the cathode. Explicitly, for the150 nm Ni-cathoded cell, the J_(sc) decreased from 22.4 mA/cm⁻² on day 1to 20.4 mA/cm⁻² on day 5, and FF increased from 43% on day 1 to 50% onday 5. For the 300 nm Ni-cathoded cell, V_(oc) changed from the 0.70 Von day 1 to 0.75 V on day 5, J changed slightly from 20.1 mA/cm⁻² on day1 to 20.6 mA/cm⁻² on day 5, and FF increased from 50% on day 1 to 61% onday 5. We think that the decrease in J_(sc) for the 150 nm Ni-cathodedcell is likely due to the absorption of moisture in the CH₃NH₃PbI₃. Theoverall efficiencies increase from 6.7% on day 1 to 7.7% on days for the150 nm Ni-cathoded cell and 7.8% on day 1 to 10.4% on day 5 for the 300nm Ni-cathoded cell. For comparison, the results for cells with 100 nmnickel cathode are shown in FIG. 53 ²¹ and Table S1²².

We further studied the photovoltage rising transients of bothgold-cathoded and nickel-cathoded cells. The photovoltage risingtransient reflects how rapidly the electrons and holes can bephotoinduced and further transport through the internal layers(electrons from perovskite→TiO₂→FTO and holes from perovskite→HTM→metals(gold or Ni)) in the presence of the recombination of the aboveprocess.²⁰ FIG. 7 shows that the rising time of V_(oc) (defined as thetime it takes from shuttle open to 90% of the full V_(oc)) for theAu-cathoded and Ni-cathoded cells are 27.5 and 31.9 ms, respectively.This result suggests that replacement of gold cathode with Ni cathodedoes not significantly vary the charge transport kinetics. Therefore,nickel is a potentially suitable low-cost metal as cathode in perovskitesolar cells. Note that, neither the motion of the shutter (responsetime=0.7 ms from full-close to full-open) nor the data acquisition rate(with the highest rate at 10 μs per data point) is the bottleneck of theabove measurement, as confirmed by a commercial ultrafast siliconphotodiode (see Figure S4 in Supporting Information²¹).

TABLE 3 Photovoltaic Performance of Solar Cells with Ni as CathodeMeasured on Different Days^(a) J_(sc) V_(oc) Cell Name (mA m⁻²) (mV) FFη (%) 150 nm Ni Day 1 22.4 702 0.43 6.7 Day 2 22.2 752 0.45 7.5 Day 520.4 752 0.50 7.7 300 nm Ni Day 1 21.0 746 0.50 7.8 Day 2 20.6 822 0.6010.2 Day 5 20.6 830 0.61 10.4 ^(a)Between each measurement, the devicesare stored in a desiccator.

The accelerated stability tests for perovskite materials were performedby monitoring the reflection of the corresponding perovskite films in95% humidity air at room temperature. Increase of reflection in FIG. 8afor the conventional CH₃NH₃PbI₃ is due to the decomposition ofperovskite structure in moisture. As can be seen, conventionalperovskite CH₃NH₃PbI₃ started to decompose immediately after beingexposed to moisture. After 1.5 hours, most of the black film has beendecomposed to the yellowish color, originated form PbI₂, and thecorresponding reflection increases from about 10% to about 20%. In thecase of CH₃NH₃Pb(SON)₂I as shown in FIG. 3b , no obvious reflectionincrease was observed even after 4 hours exposure to moisture. Thecorresponding reflection increases only 2% after 4 hour of exposure inmoisture, from about 6% to 8% and a tiny white piece emerges at thecorner, which is likely due to Pb(SCN)₂. In addition, the black color ofCH_(a)NH₃Pb(SCN)₂I film can last for months in air with humidity 40% orbelow. The lower inserted images in FIGS. 8a and 8b compare theCH₃NH₃PbI₃ film and CH₃NH₃Pb(SCN)₂I film after 30 days in air at 20˜40%humidity. According to the crystal structure of CH₃NH₃PbX₃, perovskitematerial, ligands between lead and halogen atoms form the frame of theperovskite structure. The first step of degradation in moisture involvesa formation of a hydrated intermediate containing isolated PbX₆ ⁴⁻octahedral The formation constants K₄ is calculated to be only 0.35 forPbI₄ ²⁻, which corresponds to the weak interaction between halogens andlead. In the case of CH₃NH₃Pb(SCN)₂I, the interaction between lead andthiocyanate is much stronger and the formation constant K₄ is up to 7for Pb(SCN)₄ ²⁻. Comparing the spheric shape I′ with the linear shapeSCN⁻ as indicated by its Lewis structure, the lone pair of electronsfrom S and N ire SCN⁻ can form stronger ligand with Pb, which in turnstabilizes the frame structure of CH₃NH₃Pb(SCN)2I. We show that directimprovement of moisture tolerance for CH₃NH₃Pb(SCN)₂I has been observedin accelerated stability test compared with traditional CH₃NH₃PbI₃perovskite material.

FIG. 9 collects the X-ray diffraction (XRD) patterns of perovskite typeCH₃NH₃Pb(SCN)₂I in comparison to the conventional CH₃NH₃PbI₃ film, alongwith Pb(SCN)₂ and CH₃NH₃I (MAO. The typical characteristic XRD peaks(2θ) for the conventional perovskite CH₃NH₃PbI₃ at 14, 20, 29, 32, 41degree, ail present as expected. In comparison, these peaks are alsofound for CH₃NH₃Pb(SCN)₂I, which strongly indicates that ourCH₂NH₃Pb(SCN)₂I material is also in perovskite structure. The calculatedXRD pattern for CH₃NH₃Pb(SCN)₂I matches well with the major XRD peaksfrom experiment. XRD patterns for Pb(SCN)₂ and MAI were also listed inFIG. 2 as references.

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1-24. (canceled)
 25. A photovoltaic device, comprising: (1) a firstconductive layer, (2) an electron blocking layer, on the firstconductive layer, (3) a semiconductor layer, on the electron blockinglayer, (4) a light-harvesting material, on the semiconductor layer, (5)a hole transport material, on the light-harvesting material, and (6) asecond conductive layer, on the hole transport material, wherein thelight-harvesting material comprises a perovskite absorber containing apseudohalogen.
 26. The photovoltaic device of claim 25, wherein thelight-harvesting material comprises CH₃NH₃Pb(SCN)₂I.
 27. Thephotovoltaic device of claim 25, wherein the first conductive layer istransparent.
 28. The photovoltaic device of claim 25, wherein thelight-harvesting material comprises CH₃NH₃PbY₂X, wherein Y is SCN, and Xis selected from the group consisting of F, Cl, Br, or I.
 29. Thephotovoltaic device of claim 25, wherein the light-harvesting materialcomprises a lead compound.
 30. The photovoltaic device of claim 29,wherein the light-harvesting material comprises CH₃NH₃PbX₃ orH₂NCHNH₂PbX₃, and X is selected from the group consisting of F, Cl, Br,or I.
 31. The photovoltaic device of claim 25, wherein the semiconductorlayer comprises TiO₂.
 32. The photovoltaic device of claim 25, whereinthe hole transport material comprises spiro-MeOTAD.
 33. The photovoltaicdevice of claim 25, wherein the first conductive layer comprises atleast one transparent conductor selected from the group consisting ofindium-tin oxide, fluorinated tin oxide and aluminum-zinc oxide.
 34. Thephotovoltaic device of claim 25, wherein the semiconductor layercomprises TiO₂ nanowires.
 35. The photovoltaic device of claim 34,wherein the TiO₂ nanowires have a length of at most 1000 nm.
 36. Thephotovoltaic device of claim 25, wherein the second conductive layercomprises nickel.
 37. The photovoltaic device of claim 36, wherein thesecond conductive layer comprises nickel having a thickness of at least150 nm.
 38. The photovoltaic device of claim 36, wherein the secondconductive layer comprises nickel having a thickness of at least 300 nm.39. The photovoltaic device of claim 25, wherein the photovoltaic devicehas a power conversion efficiency of at least 7.7%.
 40. The photovoltaicdevice of claim 25, wherein the photovoltaic device has a powerconversion efficiency of at least 10.4%.
 41. A method of forming thephotovoltaic device of claim 25, comprising: forming the firstconductive layer, and optionally the blocking layer on the firstconductive layer; forming the semiconductor layer, on the firstconductive layer; applying the light-harvesting material, onto thesemiconductor layer; forming the hole transport layer, on thesemiconductor layer; and forming the second conductive layer, on thehole transport layer.
 42. The method of claim 41, wherein forming thesemiconductor layer comprises a solvothermal method.
 43. The method ofclaim 41, wherein forming the second conductive layer comprisessputtering or evaporation.
 44. The method of claim 41, wherein applyingthe light-harvesting material comprises spin coating.